1. Field of the Invention
The present invention generally relates to a method and apparatus for sensing small fluid samples in a vessel and, more particularly, to a method and apparatus for determining the volume of a fluid in a vessel, such as a test tube, using a bottom sensing device (e.g., tip jam device).
2. Background Description
Analyses of fluids, especially bodily fluids such as urine and blood, are important to the diagnoses and treatment of various illness and other conditions. These illnesses and conditions can range from various forms of cancers to blood diseases to drug use and others.
In order to analyze a bodily fluid, a sample of fluid is first taken from a person and analyzed either by hand or by an automatic analyzer or other device of the type well known in the art. In the case of an automatic analyzer, for example, the bodily fluid is disposed in a tube which, in turn, is disposed on a carousel or other conveying mechanism. The carousel or other conveying mechanism conveys the tube though scanning stations, for example, and under a pipetting station in order for a pipetting probe to aspirate a sample of the fluid.
The pipetting probe is then lowered into the tube in order to aspirate a sample of the fluid. Thereafter, and depending on the specific test or tests to be performed on the fluid, a specific reagent may be combined with the fluid in order for a chemical reaction to occur. This chemical reaction is then analyzed to determine, for example, the amount of analyte in a sample of fluid.
It is not uncommon for many different tests to be performed on the sample fluid using different reagents. However, in order for the appropriate tests to be performed on the sample fluid a sufficient amount of the sample fluid must be present in the tube. Accordingly, when using automatic analyzers, the sample level in the tube is normally determined by the pipette probe which is connected to a sample sensing means such as a capacitive or conductive circuit. The sensing means is triggered upon contact of the pipette tip with the surface of the sample. The pipette probe is then further lowered a distance into the sample sufficient to allow withdrawal of the required volume. However, to ensure that an insufficient volume of sample will not be drawn, owing to the sample level being too close to the bottom of the tube, the pipette tip will only be allowed to be lowered to a certain level within the tube resulting in a volume of sample, called the dead volume, that is unavailable for testing. The maximal distance the pipette tip is allowed to be lowered, and thus the nominal dead volume, is set by the manufacturer of the automatic analyzer It is noted that the actual dead volume is variable and is dependent on several dimensional tolerances that exist within and between different instruments. A more dimensionally precise automatic analyzer would allow the pipette tip to aspirate fluids at a lower level than other less precise automatic analyzers, and thereby allow the manufacturer to set a smaller sample dead volume. Any amount of sample fluid below the preset dead volume level can not be utilized to perform a test or tests thereon despite the fact that the fluid in the test tube below the dead volume level may still be sufficient to perform a test or tests thereon.
Thus, what is needed is a system that determines the exact volume of a fluid below a threshold level which may be defined as a near bottom tube level. It is noted that the near bottom tube level is an arbitrary level of fluid in the tube, and may be predefined by the manufacturer of an automatic analyzer. The determination of the exact volume of a fluid in the tube will allow the automatic analyzer or other device to determine whether there is a sufficient amount of fluid in the tube in order to perform a certain predetermined test or tests.